Endangered wetlands function as carbon sinks

As sea levels rise and climate patterns shift, one of the earth’s most sensitive environments, the margins between sea and land from which our ancestors crawled out of the seas, is drawing considerable scientific interest.

Today we feature the results of two such studies, one revealing the vital role played by those boundary zones in containing earth’s potentially lethal atmospheric carbon levels and the other focusing on the threat to wetlands posed by climate change.

This figure illustrates the efficiency of (L-R) mangrove forests, salt marshes and seagrass beds as reservoirs for carbon. More carbon dioxide is taken up from the atmosphere (green arrows) than is re-released (black arrows), while a substantial amount is stored in soils (red arrows) for hundreds to thousands of years if left undisturbed. Image Credit: Howard et al., 2017, Frontiers in Ecology and the Environment

Coastal Wetlands Excel at Storing Carbon

In the global effort to mitigate carbon dioxide levels in the atmosphere, all options are on the table—including help from nature. Recent research suggests that healthy, intact coastal wetland ecosystems such as mangrove forests, tidal marshes and seagrass meadows are particularly good at drawing carbon dioxide from the atmosphere and storing it for hundreds to thousands of years.

Policymakers are interested to know whether other marine systems—such as coral reefs, kelp forests, phytoplankton and fish—can mitigate climate effects. A new analysis co-authored by a University of Maryland scientist suggests that, while coastal wetlands serve as effective “blue carbon” storage reservoirs for carbon dioxide, other marine ecosystems do not store carbon for long periods of time.

The research paper [open access], published February 1, 2017 in the journal Frontiers in Ecology and the Environment, also notes that coastal wetlands can help protect coastal communities from storm surges and erosion. Coastal wetland areas are easier for governments to manage compared with ecosystems that reside in international waters, further adding to the strategic value of coastal wetlands in the fight against climate change.

“We compared many different coastal ecosystems and have made a clear case for including coastal wetlands in discussions about greenhouse gas mitigation,” said Ariana Sutton-Grier, an assistant research scientist at UMD’s Earth System Science Interdisciplinary Center and a co-lead author of the research paper. “Coastal wetlands store a lot of carbon in their soils and are important long-term natural carbon sinks, while kelp, corals and marine fauna are not.”

The research paper integrates previous data on a variety of coastal and marine ecosystems to determine which systems are best suited to mitigate climate effects. To make this assessment, Sutton-Grier and her colleagues evaluated how effectively each ecosystem captures carbon dioxide—for example, by plants using it to build their branches and leaves—and how long the carbon is stored, either in plant tissues or in soils.

Coastal wetlands outperformed other marine systems in just about every measure. For example, the researchers estimated that mangrove forests alone capture and store as much as 34 million metric tons of carbon annually, which is roughly equivalent to the carbon emitted by 26 million passenger cars in a year. Estimates for tidal marshes and seagrass meadows vary, because these ecosystems are not as well mapped globally, but the total for each could exceed 80 million metric tons per year.All told, coastal wetlands may capture and store more than 200 metric tons of carbon per year globally. Importantly, these ecosystems store 50-90 percent of this carbon in soils, where it can stay for thousands of years if left undisturbed.

“When we destroy coastal wetlands, for coastal development or aquaculture, we turn these impressive natural carbon sinks into additional, significant human-caused greenhouse gas sources,” said Sutton-Grier, who is also an ecosystem science adviser for the National Ocean Service at the National Oceanic and Atmospheric Administration.

The researchers’ goal is to help inform resource managers and policymakers where to focus their limited resources to have the greatest impact on climate mitigation. The new analysis acknowledges that other ecosystems, such as coral reefs and kelp forests, provide valuable storm and erosion protection, key fish habitat and recreation opportunities, and thus deserve protection. But their capacity to store carbon over the long term is limited.

“A common question I get from coastal managers and other stakeholders is whether oyster reefs, coral and kelp are effective ‘blue carbon’ habitats,” said Stefanie Simpson, a co-author of the paper and manager of the Blue Carbon program at the nonprofit organization Restore America’s Estuaries. “This paper highlights the role all of these ecosystems have in the carbon cycle, while calling out our coastal habitats—marsh, seagrass and mangroves—for their role as significant and long-term carbon stores.”

Researchers have often looked to terrestrial forests as carbon sinks as well. But most forests do not store substantial amounts of carbon in their soils. As such, the researchers believe that coastal “blue carbon” habitats may stand alone as the most efficient biological reservoirs of stored carbon on Earth.

“The concept of ‘blue carbon’ has focused scientists and stakeholders on the tremendous potential of managing marine ecosystems for climate mitigation,” said Patrick Megonigal, associate director for research at the Smithsonian Environmental Research Center, who reviewed an early draft of the manuscript but was not directly involved in the work. “This analysis takes a big step forward by explaining why coastal wetland ecosystems are particularly attractive for carbon-based management.”

Changes in Rainfall, Temperature Expected to Transform Coastal Wetlands This Century

Sea-level rise isn’t the only aspect of climate change expected to affect coastal wetlands: changes in rainfall and temperature are predicted to transform wetlands in the Gulf of Mexico and around the world within the century. These changes will take place regardless of sea-level rise, a new study from the US Geological Survey and the University of Texas Rio Grande Valley concludes.

Such changes are expected to affect the plant communities found in coastal wetlands. For example, some salt marshes are predicted to become mangrove forests, while others could become salty mud flats. These shifts in vegetation could affect the ecological and economic services wetlands provide to the communities that rely on them.

“Coastal wetlands are an invaluable resource,” said Christopher Gabler, a former USGS scientist, currently an assistant professor at the Texas university, and lead author of the study, published January 23 in Nature Climate Change. “They protect surrounding communities from storms and coastal erosion, support fisheries and wildlife, purify water pollution, and help prevent dead zones from forming in the Gulf.”

It’s unknown exactly how these services would be affected as wetlands transform in response to climate change, which has many different facets. Though studies on climate change impacts in wetlands have typically addressed sea-level rise, this research looked at aspects of climate change that have received little attention.

“Most studies have focused on the impact of sea-level rise on coastal wetlands and have excluded the important role of temperature and precipitation,” said Michael Osland, a USGS research ecologist and study co-author. “We know that climate influences how these wetlands look and work, so this study aimed to demonstrate the importance of considering these forces when modeling what coastal wetlands may look like in the future.”

The intensive study relied on field studies at 10 estuaries in five states (Texas, Louisiana, Mississippi, Alabama, and Florida) along the northern Gulf of Mexico. The fieldwork took place in a variety of coastal wetland types, including mangroves, marshes, and salt flats.

The model included the current climatic conditions, which were based on climate observations from the northern Gulf of Mexico for the years 1981 to 2010. The researchers then evaluated how coastal wetlands might be affected by several potential future climates, including temperature increases of 3.6 to 7.2 degrees Fahrenheit (2 to 4 degrees Celsius) by 2100 and a 10% increase or 10% decrease in rainfall from current levels.

“The Gulf of Mexico is one of the best natural laboratories in the world for studying the influence of temperature and precipitation on coastal wetlands,” Osland said. “Coastal wetlands function similarly worldwide, so our results in this region also indicate what the potential future changes could be in other areas with coastal wetlands.”

The predicted changes are likely to change how these wetlands operate, Gabler added. But it’s not all bad news.

“There are going to be winners and losers,” he said. “Some things are going to be better; some are going to be worse. The systems will do things differently, and ‘different’ is a challenge.”